1. Field of the Invention
The present invention relates to a wire rod steel, that is, a steel product (such as a wire rod or a steel bar) which is used in feeding rollers or paper ejecting rollers (such as various paper feed rollers and paper money feed rollers) which are used in printers or copy machines, out of high-precision and high speed conveying rollers used in industrial machinery; and a process for producing the same. The present invention relates particularly to a steel product superior in straightness after being subjected to cold drawing; and a process suitable for producing such a steel product.
2. Description of Related Art
As development in high-speed printing and multicolored printing in copy machines or printers has been advancing in recent years, a high paper feed precision has been coming to be regarded as important. For this reason, a high precision is demanded for paper feed rollers or paper ejecting rollers in industrial machinery.
In case of producing such a paper feed roller or a paper ejecting roller as above, a steel bar prepared by subjecting a wire-form rolled steel to cold drawing and straightening is cut into a given length to form a round bar as a roller axial part. In such a production process, various techniques have been hitherto suggested in order to improve the paper feed precision of the roller axial part.
As such a technique, for example, JP-A-11-20962 discloses a method of forming spike-form projections on a surface of a round rod made of a metal by plastic working to heighten grip ability between the rod and paper, thereby improving a paper feed precision. JP-A-10-329971 discloses a method of adhering an abrasive grain of alumina, silicon carbide or the like onto the surface of a round rod made of a metal to heighten grip ability between the rod and paper, thereby improving a paper feed precision. JP-A-8-301496 discloses a method of covering the surface of a round rod made of a metal with a rubber to heighten grip ability between the rod and paper, thereby attaining high-precision paper feed ability.
However, in all of the above-mentioned methods suggested until now, only the surface quality of a round rod is adjusted and no characteristic of the round rod which is a raw material is investigated. In other words, a basically important characteristic for improving a paper feed precision is that a round rod used as a roller axial part has straightness, that is, the straightness of the rod after cold drawing is high. Inventions made by investigations from such a standpoint are hardly suggested under the actual situations. As such an invention, only JP-A-4-168244 discloses a method of fixing nitrogen in steel for mechanical structure use, as AlN, with Al to reduce solid solution nitrogen, thereby improving straightness.
Under the above-mentioned situations, the present invention has been made. An object thereof is to provide a wire rod steel product making it possible to improve straightness (i.e., straightness after cold drawing) of a roller axial part necessary for attaining a high paper feed precision of a paper feed roller or a paper ejecting roller; and a process suitable for producing such a steel product.
The hot-rolled wire rod of the present invention which can attain the above-mentioned object is a wire rod steel superior in straightness, after being subjected to cold drawing, comprising: C: 015% or less (not including 0%), Si: 0.05% or less (not including 0%), Mn: 0.3-2%, P: 0.2% or less (not including 0%), S: 0.08-0.5%, Al: 0.05% or less (not including 0%), N: 0.01% or less (not including 0%), inevitable impurities, and Fe as the balance, the ferrite grain size number of the wire rod steel being No. 11 or less according to ISO 643. Herein, xe2x80x9c%xe2x80x9d means xe2x80x9cmass %xe2x80x9d. The same rule applies correspondingly to the following in the specification, which does not include claims.
The wire rod steel is directed mainly to various free-cutting steels, and has good machinability on the basis of the above-mentioned composition.
If necessary, the wire rod steel of the present invention may comprise one or more selected from the group consisting of Pb: 0.4 mass % or less (not including 0 mass %), Bi: 0.4 mass % or less (not including 0 mass %), Te: 0.2 mass % or less (not including 0 mass %), Se: 0.2 mass % or less (not including 0 mass %) Sn: 0.4 mass % or less (not including 0 mass %), and In: 0.4 mass % or less (not including 0 mass %)
In case of producing the wire rod steel of the present invention, preferably, hot working of the wire rod steel is ended at 80xc2x0 C. or higher, and subsequently the wire rod steel is cooled at a cooling rate of 3.0xc2x0 C./second or less within the temperature range of 800-600xc2x0 C. ISO 643 completely corresponds to JIS G 0552. Therefore, ferrite grain size number according to ISO 643 is the same number as that according to JIS G 0552.